Mammalian thermoregulation is typically divided into two components, autonomic and behavioral. The characteristics of both components are well known in adults, but their coordination in adults makes it difficult to discern the ways in which animals integrate and coordinate responses. Developmental approaches are useful in this regard because altricial newborn mammals possess immature responses that become mature and coordinated during relatively brief developmental transitions. One such transition, recently identified in this laboratory, involves a diminution in thermoregulatory responsiveness between the ages of two and eight days. This transition is important because its occurrence suggests mechanisms of thermoregulatory activation that are different from those currently suspected. Specifically, it is believed that heat production in newborn rats is activated solely by direct neural activation of brown adipose tissue (BAT). It is hypothesized here, however, that circulating catecholamines, released from chromaffin tissue in the adrenal medulla and/or the organ of Zuckerkandl, contribute to BAT activation. It is further hypothesized that the transition from robust BAT activation at birth to muted BAT activation within one week results from developing sympathetic innervation as well as involution of the organ of Zuckerkandl. These hypotheses will be tested using pharmacological and surgical manipulations of these organs and systems during development. In parallel with these physiological investigations, a novel head-turning paradigm involving radiant heat reinforcement will be used to investigate the development of thermal preference over the same postpartum period. In addition, because a synthesis of these two approaches is one primary goal of this project, interactions between autonomic and behavioral responses will be investigated as pups develop. It is hypothesized that coordination of physiological and behavioral responses will emerge as both thermoregulatory components mature during the first week postpartum. Finally, these interdisciplinary studies of week-old rats may provide valuable information regarding the comparable developmental stage of two to four months in human infants, at which time infants are most at risk for Sudden Infant Death Syndrome.